1. Field of the Invention
The present invention relates to a process for producing vinyl ester polymer and a process for producing vinyl alcohol polymer. More particularly, it pertains to a process for producing vinyl ester polymer having a high degree of polymerization by means of seed polymerization or emulsion polymerization, a process for producing vinyl alcohol polymer having a high degree of polymerization by the use of said vinyl ester polymer obtained by the above-mentioned process, and a process for producing vinyl ester polymer emulsion well suited for use as a seed emulsion.
2. Description of the Related Arts
Vinyl ester polymer, especially vinyl acetate polymer is widely utilized as a base polymer of adhesives and paints, and is also of great importance as a raw resin of vinyl alcohol polymer. It is also well-known that vinyl alcohol polymer is not only utilized as a paper processing agent, textile processing agent, emulsion stabilizer, etc. but also assumes an important place as a raw material of vinylon film and vinylon fiber by virtue of its surpassing interfacial characteristics and strength properties as one of the crystalline water-soluble polymers limited in number.
Conventional vinyl alcohol polymer, however, is limited in the degree of polymerization, with an upper limit of 2000, and 3000 approximately in the case of a special product by reason of the fact that vinyl alcohol polymer having a high degree of polymerization is poor in processability and hard to handle and that vinyl acetate polymer as a raw material having a high degree of polymerization is difficult to obtain.
The recent rapid innovation in processing technique, however, has made possible the processing of polymers having a ultrahigh degree of polymerization thereby leading to success in deriving unique physical properties which have never been known. This is also the case with vinyl alcohol polymer, and that given a high degree of polymerization is expected to exhibit improved characteristic properties in the conventional application areas and also to find new uses in novel fields such as high tenacity fibers, polarizing film, etc.
As a general rule, a polymer having a high degree of polymerization is obtained by polymerization at a low temperature and at a low polymerization rate. This holds true for vinyl acetate, and several cases have been reported, (For example, A. R. Shultz; J. Am. Chem. Soc. 76, 3422(1954), G. M. Burnett, M. H. George, H. W. Melville; J. Polym. Sci. 16, 31(1955), and M. Matsumoto, Y. Ohyanagi; J. Polym. Sci, 26, 148(1960)).
However, the above-mentioned conventional processes are based on bulk polymerization, which involves several problems still unsolved such as difficulty in stirring due to high viscous polymerization system making the production of uniform polymer impossible, difficulty in removing the heat of polymerization, etc. Therefore, the industrial scale production by bulk polymerization is considered to be almost impossible.
In order to overcome the disadvantage of bulk polymerization, there is proposed a new process by suspension polymerization (Japanese Patent Application Laid-Open No. 148209/1986).
However, in a system with great chain transfer such as vinyl ester system, it is indispensable to lower the polymerization temperature for the purpose of increasing the degree of polymerization. Because of thus, the rate of polymerization is inevitably made quite low in bulk polymerization and suspension polymerization according to the reports hereinbefore.
There is also proposed another process, that is, low temperature emulsion polymerization as a process having overcome the disadvantages of the above polymerization processes, which process is different from bulk polymerization and suspension polymerization in polymerization mechanism, almost free from the problems of stirring and heat removal even at a relatively high rate of polymerization, and makes it possible to obtain vinyl ester polymer and vinyl alcohol polymer each having a high degree of polymerization (Japanese Patent Application Laid-Open No. 37106/1988).
Nevertheless, there still remain some problems in the aforementioned low temperature emulsion polymerization that a long time is required from the beginning of an initiator addition to the start of polymerization, the ratio of weight-average molecular weight of vinyl alcohol polymer having a high degree of polymerization obtained (Mw) to number-average molecular weight of the same (Mn) (Mw/Mn) is high, which means a wide range of molecular-weight distribution, and the vinyl alcohol polymer thus obtained has an average degree of polymerization of only 25,000 at the highest.
There is also known a polymerization process in which 0.01 to 0.6% by weight of an anionic emulsifier is added to vinyl chloride monomer, and prior to the completion of polymerization, at least 0.2% by weight of an nonionic emulsifier is added to the monomer to complete emulsion polymerization and produce an emulsion, which is then used as a seed emulsion for seed-polymerizing vinyl chloride monomer (Japanese Patent Application Laid-Open No. 60389/1979). The process, however, is directed to the process for producing a vinyl chloride polymer emulsion having a sharp particle size distribution and large particles and is incapable of producing a vinyl ester polymer having a high degree polymerization.
There is further known a polymerization process in which ethylene and vinyl acetate are subjected to continuous emulsion polymerization by using vinyl acetate polymer emulsion or ethylene-vinyl acetate copolymer emulsion each obtained by emulsion polymerization at 40.degree. to 75.degree. C. (Japanese Patent Application Laid-Open No. 145783/1979). However, the process fails to produce a vinyl ester polymer having a high degree of polymerization.
There is further known a polymerization process in which a monomer of vinyl acetate or acrylic acid is subjected to emulsion polymerization at 50.degree. to 90.degree. C. to produce an emulsion, which is then used as seed emulsion for seed-polymerizing ethylene, styrene, acrylic acid, etc. at 50.degree. to 90.degree. C. and at an initiator charge of 5.0.times.10.sup.-4 to 5.0.times.10.sup.-3 mol per unit volume (1 liter) of reactant liquid charge (Japanese Patent Application Laid-Open No. 103489/1979). The process, however, is incapable of producing a vinyl ester polymer having a high degree of polymerization because of a large amount of initiator charge required and a high polymerization temperature.